Lights boxes have been known and used in the past for backlit illumination of displays and transparencies. The previously known light boxes or backlit display devices relied upon fluorescent tubular lamps to provide the backlighting illumination. The transparencies were mounted on thin sheets of plastic or acrylic material which is typically a white opaque or a clear sheet of acrylic plastic, about 1/16 to 1/4 of an inch thick (about 1.5-6.5 mm). A transparency having pictures, symbols, or writings thereon could be mounted on the plane of the acrylic sheet, such that illumination from behind allowed the images to be viewed without reflective lighting. Such display devices are highly beneficial for use in low lighting indoor situations or where outdoor visibility is required during the night. Such devices have been useful as advertising displays, signage, and informational directories in restaurants, retail establishments, office buildings, and the like.
Backlighting using incandescent light bulbs or lamps has not been popular because of the high heat generation of incandescent light bulbs. Thus, light boxes have typically used standard fluorescent tubular light bulbs or lamps usually in the form of parallel, straight fluorescent tubes which are about 11/2 inch in diameter (about 3.8 centimeters diameter) which are spaced apart horizontally or vertically along the rear of the light box to form a substantially even lighting plane. Circular fluorescent tube lamps have been used in some cases where the display area is small. In order to avoid observable bright lines through a plastic transparency along each fluorescent tube (such bright lines are commonly referred to as "hot spots"), the light plane must be spaced apart a sufficient distance from the transparency to allow the light from each tube to adequately diffuse before illuminating the transparency. Even for opaque plastics or clear plastic with a diffusion coating or a thin paper covering, each of which provide a certain amount of light diffusion, the minimum diffusion distance from the bulb surface to avoid hot spots is in the range of about 3/4 of an inch to 11/2 inches (about 2 centimeters to about 4 centimeters). As a result of the combined fluorescent tube diameter and the required diffusion distance, as well as construction or structural requirements of the light boxes, previously known light boxes have been about 4 or 5 inches deep (about 10 to 13 centimeters) with the minimum previously known light box of the type using standard fluorescent tubes, about 31/4 inches deep.
The transformers used in previously known light boxes were typically the standard magnetic coil ballasts for fluorescent lighting, which are generally bulky, heavy units which convert standard house current such as 120 volts AC at 60 cycles per second into 700-800 volts AC at 60 cycles. Spacing requirements for such transformer ballasts could usually be accommodated by the 4 or 5 inch deep light boxes. However, in the case of some thin light boxes with a depth of about 31/4 inches, the spacing requirements as well as the structural requirements caused the cost of manufacturing such units to be increased compared to deeper light boxes. Typical life span for fluorescent lamps is about 2,000-4,000 hours of continuous illumination. The life span is shorter in illuminated hours when the units are turned on and off frequently and failures may be expected after about 1,000 starts, or less.
There has been a need for thin light boxes which, for example, can be wall mounted without recessing the box into the wall. However, entirely adequate solutions to the various problems associated with constructing a thin light box have not previously been known. One thin light box using a large number of very small diameter fluorescent tube lamps has been produced. The resulting light box has a depth of approximately 13/4 inches, but is very expensive to manufacture and maintain. These light boxes use small diameter straight fluorescent lamp tubing, approximately 1/4 of an inch (about 6 mm) in diameter. The thin profile of these light boxes also requires a special electronic ballast to produce the appropriate voltage required for the fluorescent tubular lamps. The typical life span for the very small diameter fluorescent lamps can be less than about 500 hours of continuous use and shorter for frequent starts. This is particularly disadvantageous where a larger number of small diameter fluorescent lamps are required to provide the same intensity of illumination as provided with the larger diameter fluorescent bulbs.